Intercoupled lengths of tubular elements are used throughout the international oil production industry, with a high proportion of such installations being based on standards set by the American Petroleum Institute (API). For example, although there are a number of premium tubular products used for specialized or particularly difficult applications, the majority of the production tubing and casing tubular goods, including couplings, in use are made up of standardized types, such as those with API 8 round or buttress threads. Stocks of Internally Plastic Coated and Fiberglass Lined tubing and casing are now widely employed, especially where production wells are located in more environmentally demanding locations. The standard tubing sections are interconnected by internally threaded standardized cylindrical sleeves or couplers, also, if required, internally plastic coated. In the petroleum industry, product interchangeability is essential for economy where feasible because strings are made up and disassembled as many times as conditions will permit. Thus standardization, under API criteria as to sizes, materials, thread types, and tolerances, enables widespread use of tubing and casings which, can be replaced and interchanged multiple times economically during their useful lives.
Usually after manufacture, a reserve of tubing or casing is held in inventory in a pipe yard or on a drilling or production site. One end of a pipe length, called the mill end, typically is pre-attached to a coupling sleeve, for storage and shipment to at a production site when needed. At the production site, the string is assembled by stabbing the pin (“field”) end of a different pipe length into the available mill end of a coupler, which is disposed vertically as the new length is engaged, rotated, and made up to proper API torque specifications. The string is built up and successively fed downhole until the production depth is reached. As production depths are historically consistently increasing, in order to gain access to new oil and natural gas sources, greater stresses and physical demands, are concomitantly being imposed on the tubular goods and especially on the threaded connections. Thus, it is now common to employ seal elements within couplers to engage each of the opposite tubing pin ends in order to seal against interior fluid and pressures, and to combat leakage.
Tubing strings must periodically be withdrawn for service, inspection or replacement of problem components, so that tubing strings commonly require repeated make-ups. Seals used in the string must therefore also be reusable as many times as feasible. In the present state of the art, the pipe strings are extremely long and must withstand not only high pressures and temperatures but also harsh and corrosive environments. Accordingly, minor irregularities and non-uniformities in the seals, the plastic coating or the tubing can be the source of major and costly problems.
While modern plastic coating methods are efficient, they do not always assure uniformity in critical thread regions. For example coating layer discontinuities can often appear in pin end regions, because uniform deposition of coating is more difficult in such transition regions. It is, in other words, sometimes dubious, as to whether available manufacturing procedures have provided a uniform corrosion barrier and effective sealing against high internal pressures, even where an internal seal has been employed.
Moreover, there are certain factors inherent in synthetic materials used in tubing connections which render seals formed from such materials susceptible to failure when exposed to high pressure gases, particularly under elevated temperatures. “Teflon” is a material widely used for petroleum seals, because of its chemical stability and resilient properties. The chemistry of “Teflon” and other plastic formulations, however, is such that pressurized gases will, with time, permeate into and throughout the seal. When there has been sufficient exposure to such permeating gases, a seal may in time become fully permeated and begin to distort in shape. If the internal pressure is reduced or relieved, the gas-permeated seal is apt to implode into the tubing string or distort in shape, losing its sealing capability. This loss of seal integrity requires removal and repair of the entire string. Accordingly, there exists a need for a superior seal/corrosion barrier and coupling combination which can meet the aggravated and demanding conditions imposed by high pressure, high temperature, corrosive downhole environments.